net/sched: sch_ets: don't remove idle classes from the round-robin list

In the Linux kernel, the following vulnerability has been resolved:

net/sched: sch_ets: don't remove idle classes from the round-robin list

Shuang reported that the following script:

1) tc qdisc add dev ddd0 handle 10: parent 1: ets bands 8 strict 4 priomap 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7
2) mausezahn ddd0 -A 10.10.10.1 -B 10.10.10.2 -c 0 -a own -b 00:c1:a0:c1:a0:00 -t udp &
3) tc qdisc change dev ddd0 handle 10: ets bands 4 strict 2 quanta 2500 2500 priomap 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3

crashes systematically when line 2) is commented:

list_del corruption, ffff8e028404bd30->next is LIST_POISON1 (dead000000000100)
------------[ cut here ]------------
kernel BUG at lib/list_debug.c:47!
invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
CPU: 0 PID: 954 Comm: tc Not tainted 5.16.0-rc4+ #478
Hardware name: Red Hat KVM, BIOS 1.11.1-4.module+el8.1.0+4066+0f1aadab 04/01/2014
RIP: 0010:__list_del_entry_valid.cold.1+0x12/0x47
Code: fe ff 0f 0b 48 89 c1 4c 89 c6 48 c7 c7 08 42 1b 87 e8 1d c5 fe ff 0f 0b 48 89 fe 48 89 c2 48 c7 c7 98 42 1b 87 e8 09 c5 fe ff <0f> 0b 48 c7 c7 48 43 1b 87 e8 fb c4 fe ff 0f 0b 48 89 f2 48 89 fe
RSP: 0018:ffffae46807a3888 EFLAGS: 00010246
RAX: 000000000000004e RBX: 0000000000000007 RCX: 0000000000000202
RDX: 0000000000000000 RSI: ffffffff871ac536 RDI: 00000000ffffffff
RBP: ffffae46807a3a10 R08: 0000000000000000 R09: c0000000ffff7fff
R10: 0000000000000001 R11: ffffae46807a36a8 R12: ffff8e028404b800
R13: ffff8e028404bd30 R14: dead000000000100 R15: ffff8e02fafa2400
FS: 00007efdc92e4480(0000) GS:ffff8e02fb600000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000682f48 CR3: 00000001058be000 CR4: 0000000000350ef0
Call Trace:
<TASK>
ets_qdisc_change+0x58b/0xa70 [sch_ets]
tc_modify_qdisc+0x323/0x880
rtnetlink_rcv_msg+0x169/0x4a0
netlink_rcv_skb+0x50/0x100
netlink_unicast+0x1a5/0x280
netlink_sendmsg+0x257/0x4d0
sock_sendmsg+0x5b/0x60
____sys_sendmsg+0x1f2/0x260
___sys_sendmsg+0x7c/0xc0
__sys_sendmsg+0x57/0xa0
do_syscall_64+0x3a/0x80
entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7efdc8031338
Code: 89 02 48 c7 c0 ff ff ff ff eb b5 0f 1f 80 00 00 00 00 f3 0f 1e fa 48 8d 05 25 43 2c 00 8b 00 85 c0 75 17 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 58 c3 0f 1f 80 00 00 00 00 41 54 41 89 d4 55
RSP: 002b:00007ffdf1ce9828 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 0000000061b37a97 RCX: 00007efdc8031338
RDX: 0000000000000000 RSI: 00007ffdf1ce9890 RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000000000001 R09: 000000000078a940
R10: 000000000000000c R11: 0000000000000246 R12: 0000000000000001
R13: 0000000000688880 R14: 0000000000000000 R15: 0000000000000000
</TASK>
Modules linked in: sch_ets sch_tbf dummy rfkill iTCO_wdt iTCO_vendor_support intel_rapl_msr intel_rapl_common joydev pcspkr i2c_i801 virtio_balloon i2c_smbus lpc_ich ip_tables xfs libcrc32c crct10dif_pclmul crc32_pclmul crc32c_intel serio_raw ghash_clmulni_intel ahci libahci libata virtio_blk virtio_console virtio_net net_failover failover sunrpc dm_mirror dm_region_hash dm_log dm_mod [last unloaded: sch_ets]
---[ end trace f35878d1912655c2 ]---
RIP: 0010:__list_del_entry_valid.cold.1+0x12/0x47
Code: fe ff 0f 0b 48 89 c1 4c 89 c6 48 c7 c7 08 42 1b 87 e8 1d c5 fe ff 0f 0b 48 89 fe 48 89 c2 48 c7 c7 98 42 1b 87 e8 09 c5 fe ff <0f> 0b 48 c7 c7 48 43 1b 87 e8 fb c4 fe ff 0f 0b 48 89 f2 48 89 fe
RSP: 0018:ffffae46807a3888 EFLAGS: 00010246
RAX: 000000000000004e RBX: 0000000000000007 RCX: 0000000000000202
RDX: 0000000000000000 RSI: ffffffff871ac536 RDI: 00000000ffffffff
RBP: ffffae46807a3a10 R08: 0000000000000000 R09: c0000000ffff7fff
R10: 0000000000000001 R11: ffffae46807a36a8 R12: ffff8e028404b800
R13: ffff8e028404bd30 R14: dead000000000100 R15: ffff8e02fafa2400
FS: 00007efdc92e4480(0000) GS:ffff8e02fb600000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000000
---truncated---